Chapter23

Chapter23 - BCH 4054 Fall 2000 Chapter 23 Lecture Notes...

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Chapter 23, page 1 BCH 4054 Fall 2000 Chapter 23 Lecture Notes Slide 1 Chapter 23 Gluconeogenesis Glycogen Metabolism Pentose Phosphate Pathway Slide 2 Gluconeogenesis • Humans use about 160 g of glucose per day, about 75% for the brain. • Body fluids and glycogen stores supply only a little over a day’s supply. • In absence of dietary carbohydrate, the needed glucose must be made from non- carbohydrate precursors. • That process is called gluconeogenesis. Slide 3 Gluconeogenesis, con’t. • Brain and muscle consume most of the glucose. • Liver and kidney are the main sites of gluconeogenesis. • Substrates include pyruvate, lactate, glycerol, most amino acids, and all TCA intermediates. • Fatty acids cannot be converted to glucose in animals. (They can in plants because of the glyoxalate cycle.)
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Chapter 23, page 2 Slide 4 Gluconeogenesis, con’t. • Substrates include anything that can be converted to phosphoenolpyruvate . • Many of the reactions are the same as those in glycolysis. • All glycolytic reactions which are near equilibrium can operate in both directions. • The three glycolytic reactions far from equilibrium (large - G) must be bypassed. • A side by side comparison is shown in Fig 23.1. Remember it is necessary for the pathways to differ in some respects, so that the overall G can be negative in each direction. Usually the steps with large negative G of one pathway are replaced in the reverse pathway with reactions that have a large negative G in the opposite direction. Slide 5 Unique Reactions of Gluconeogenesis • Recall that pyruvate kinase , though named in reverse, is not reversible and has a G of –23 kJ/mol. (Table 19.1b) • Reversal requires an input of two ATP equivalents, and hence two reactions. • Pyruvate carboxylase (Fig 23.2) • Recall the biotin prosthetic group.( Fig 23.3) • PEP Carboxykinase (Fig 23.6) Slide 6 Pyruvate Carboxlyase • Already discussed as an anaplerotic reaction. • Biotin prosthetic group communicates between a biotin carboxylase and a transcarboxylase , with carboxybiotin an intermediate. (Fig 23.4) • Activated by acyl-CoA. • Located in mitochondrial matrix. Acyl-CoA activation, especially acetyl-CoA, regulates the fate of pyruvate. When acetyl-CoA is low, pyruvate is broken down by pyruvate dehydrogenase. High levels of acetyl-CoA signal the need for more OAA to run the citric acid cycle, or to be converted to glucose.
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Chapter 23, page 3 Slide 7 PEP Carboxykinase • Also mentioned as a possible anaplerotic reaction because it is reversible. • GTP is utilized, but that is equivalent to ATP. • Additonal GTP makes formation of PEP energetically favorable. • In the cytoplasm in some tissues, in the
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Chapter23 - BCH 4054 Fall 2000 Chapter 23 Lecture Notes...

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